Flexible durable cement

ABSTRACT

According to at least one embodiment of the present description, a cement slurry includes 50% to 90% BWOC of a cement precursor material based on a total weight of the cement slurry; and from 10% to 50% BWOC of a flexible additive based on the total weight of the cement slurry. The flexible additive includes hydrophilic polyolefin fibers or 2,6-di-tert-butyl-p-cresol.

TECHNICAL FIELD

Embodiments of the present description generally relate to naturalresource well drilling and, more specifically, to flexible durablecements utilized in well drilling processes.

BACKGROUND

In well drilling processes, wellbores are commonly cemented, where theannulus between the casing and the wellbore wall is filled with cement,forming a cement sheath. High internal pressure may expand the casingand the cement sheath, which causes tensile stress on the cement sheath.Generally, cement materials are brittle, and the compressive strength isgreater than the tensile strength of cement formations. Accordingly, theincreased tensile stress on the cement sheath caused by the internalpressure may cause damage, such as cracking or fracture, to the cementsheath, which may lead to undesired leaking.

The damage to the cement sheath described in the preceding paragraph maybe worsened by the high density of cement materials commonly used ascement sheaths in wellbores. Specifically, greater density cementmaterials are used in wellbores because they have less voids in thecement structures, which results in less migration of hydrocarbons fromthe geological formation into the well. However, the low number of voidsin the cement material can increase the brittleness of the cementmaterial, which may lead to damage of the cement structure when pressureis applied to the cement structure.

SUMMARY

Accordingly, there is a need for flexible additives that can be added tocement slurries to increase the flexibility of the cement materials usedin wellbores. Particularly, there is a need for flexible additives thatallow the cement material to shift when exposed to tensile stresswithout damaging the cement structure, particularly after or duringexposure of the cement to high internal pressure environments. Asreferred to in this application, high internal pressure is pressuregenerated from fracturing operations. In fracturing operations, highpressure is created by injecting fluid into a well to break geologicalformations. This pressure can also cause damage to well cement.

The present flexible additives improve damage resistance to the cementstructures in which they are included by providing a flexible polymericstructure in the cement structure, which decreases the Young's modulusand increases the Poisson's ratio of the cement. Conventional cementadditives are not able to provide the Young's modulus and Poisson'sratio achieved by the present flexible additives.

The presently described flexible additives are generally comprised ofone or more hydrophilic polyolefin fiber. In one or more embodiments,the flexible additive may be comprised of 2,6-di-tert-butyl-p-cresol.The flexible additive may be added to the cement slurry in variousamounts depending on the properties of the wellbore and the compositionand properties of the cement material. For instance, a greaterconcentration of the flexible additive may be added to cement materialsthat have greater density and a lesser concentration of the flexibleadditive may be added to cement materials that have a lesser density.The presently disclosed flexible additive may be added to the cementmaterial as a dry ingredient to the dry cement mixture, or the flexibleadditive may be added to the cement slurry.

In one embodiment, a cement slurry comprises 50% to 90% BWOC of a cementprecursor material based on a total weight of the cement slurry; andfrom 10% to 50% BWOC of a flexible additive based on the total weight ofthe cement slurry. The flexible additive comprises hydrophilicpolyolefin fibers or 2,6-di-tert-butyl-p-cresol. In some embodiments,the flexible additive consists or consists essentially of2,6-di-tert-butyl-p-cresol.

In another embodiment, a wellbore cementing system comprises: a tubularpositioned in a wellbore such that an annulus is formed between ageological formation and the tubular; and a cement structure positionedin at least a portion of the annulus. The cement structure comprisesfrom 10% to 50% by weight of the cement (BWOC) of a flexible additive,and the flexible additive comprises hydrophilic polyolefin fibers or2,6-di-tert-butyl-p-cresol. In some embodiments, the flexible additiveconsists or consists essentially of 2,6-di-tert-butyl-p-cresol.

Additional features and advantages of the described embodiments will beset forth in the detailed description which follows, and in part will bereadily apparent to those skilled in the art from that description orrecognized by practicing the described embodiments, including thedetailed description which follows as well as the claims.

DETAILED DESCRIPTION

In the present description, the following terms or units of measurementhave been abbreviated, where:

° F.=degrees Fahrenheit;

OBM=oil-based mud;

kg/m³=kilogram per cubic meter;

g/cc=grams per cubic centimeter;

BWOC=by weight of the cement;

gps=gallons per sack;

LVTD=linear variable differential transformer

psi=pounds per square inch;

rpm=rotations per minute; and

pcf=pound per cubic foot.

Embodiments of the present description are directed to flexibleadditives to be added to cement materials, such as cement slurries, andmethods of using flexible additives in cement slurries that result in acement having, among other attributes, improved tensile strength asmeasured by the Young's modulus and Poisson's ratio of the cement. Asused throughout the description, “flexible additive” refers to acomponent or mixture of components that are present in the cementmaterial or cement slurry and, when the cement has hardened into acement structure, adds flexibility to the cement structure. A “cementslurry” refers to a slurry which is cured to form a cement. In someembodiments, a cement slurry comprises 50% to 90% BWOC of a cementprecursor material based on a total weight of the cement slurry; andfrom 10% to 50% BWOC of a flexible additive based on the total weight ofthe cement slurry. The flexible additive comprises hydrophilicpolyolefin fibers or 2,6-di-tert-butyl-p-cresol. In another embodiment,a wellbore cementing system comprises: a tubular positioned in awellbore such that an annulus is formed between a geological formationand the tubular; and a cement structure positioned in at least a portionof the annulus. The cement structure comprises from 10% to 50% BWOC of aflexible additive, and the flexible additive comprises hydrophilicpolyolefin fibers or 2,6-di-tert-butyl-p-cresol. In some embodiments,the flexible additive consists or consists essentially of2,6-di-tert-butyl-p-cresol.

A wellbore is a hole that extends from the surface to a location belowthe surface. The wellbore can permit access as a pathway between thesurface and a hydrocarbon-bearing formation. The wellbore, defined andbound along its operative length by a wellbore wall, extends from aproximate end at the surface, through the subsurface, and into thehydrocarbon-bearing formation, where it terminates at a distal wellboreface. The wellbore forms a pathway capable of permitting both fluid andapparatus to traverse between the surface and the hydrocarbon-bearingformation.

Besides defining the void volume of the wellbore, the wellbore wall alsoacts as the interface through which fluid can transition between theinterior of the wellbore and the formations through which the wellboretraverses. The wellbore wall can be unlined (that is, bare rock orformation) to permit such interaction with the formation or lined (thatis, with casing, tubing, production liner or cement) so as to not permitsuch interactions.

The wellbore usually contains at least a portion of at least one tubular(that is, a fluid conduit) that links the interior of the wellbore tothe surface. Examples of such fluid conduits or tubulars include casing,liners, pipes, tubes, coiled tubing and mechanical structures withinterior voids. A fluid conduit connected to the surface is capable ofpermitting regulated fluid flow and access between equipment on thesurface and the interior of the wellbore. Example equipment connected atthe surface to the fluid conduit includes pipelines, tanks, pumps,compressors and flares. The fluid conduit is sometimes large enough topermit introduction and removal of mechanical devices, including tools,drill strings, sensors and instruments, into and out of the interior ofthe wellbore.

The fluid conduit made from a tubular usually has at least two openings(typically on opposing ends) with an enclosing surface having aninterior and exterior surface. The interior surface acts to define thebounds of the fluid conduit. Examples of tubulars and portions oftubulars used in the wellbore as fluid conduits or for making orextending fluid conduits include casing, production liners, coiledtubing, pipe segments and pipe strings. An assembly of several smallertubulars connected to one another, such as joined pipe segments orcasing, can form a tubular that acts as a fluid conduit.

When positioning a tubular or a portion of tubular in the wellbore, thevolume between the exterior surfaces of the fluid conduit or tubularportion and the wellbore wall of the wellbore forms and defines awellbore annulus. The wellbore annulus has a volume in between theexternal surface of the tubular or fluid conduit and the wellbore wall.

The wellbore contains wellbore fluid from the first moment of formationuntil completion and production. The wellbore fluid serves severalpurposes, including well control (hydraulic pressure against the fluidsin the hydrocarbon-bearing formation), wellbore wall integrity(hydraulic pressure on the wellbore wall; provides loss controladditives) and lubricity (operating machinery). Wellbore fluid is influid contact with all portions of the wellbore and everything in thewellbore that is not fluidly isolated, including the tubular internalfluid conduit, the wellbore annulus, and the wellbore wall. Other fluidconduits coupled to the wellbore often contain at least some wellborefluid.

While drilling, drilling fluid (“mud”) fills the interior of thewellbore as the wellbore fluid. Some muds are petroleum-based materialsand some are water-based materials. Petroleum-based materials compriseat least 90 weight percent of an oil-based mud (OBM). Examples ofsuitable base petroleum materials include crude oils, distilledfractions of crude oil, including diesel oil, kerosene and mineral oil,and heavy petroleum refinery liquid residues. A minor part of the OBM istypically water or an aqueous solution that resides internally in thecontinuous petroleum phase. Other OBM components can includeemulsifiers, wetting agents and other additives that give desirablephysical properties.

While performing drilling operations, wellbore fluid circulates betweenthe geological surface and the wellbore interior through fluid conduits.Wellbore fluid also circulates around the interior of the wellbore. Theintroduction of drilling fluid into the wellbore through a first fluidconduit at pressure induces the motivation for the fluid flow in thewellbore fluid. Displacing wellbore fluid through a second fluid conduitconnected to the surface causes wellbore fluid circulation from thefirst fluid conduit to the second fluid conduit in the interior of thewellbore. The expected amount of wellbore fluid displaced and returnedto the surface through the second fluid conduit is equivalent to theamount introduced into the wellbore through the first fluid conduit.Parts of the wellbore that are fluidly isolated do not supportcirculation.

The circulation and differences in movement of wellbore fluid within thewellbore can cause internal pressure of the wellbore to increase. Thisincrease in internal pressure can place stresses on the components ofthe wellbore, such as, for example, the tubular. Therefore, a cementstructure can be placed between the geological formation and thetubular.

Cementing is one of the most important operations in both drilling andcompletion of the wellbore. Primary cementing occurs at least once tosecure a portion of the fluid conduit between the wellbore interior andthe surface to the wellbore wall of the wellbore.

Primary cementing forms a protective solid sheath around the exteriorsurface of the introduced fluid conduit by positioning cement slurry inthe wellbore annulus. Upon positioning the fluid conduit in a desirablelocation in the wellbore, introducing cement slurry into the wellborefills at least a portion, if not all, of the wellbore annulus. When thecement slurry cures, the cement physically and chemically bonds withboth the exterior surface of the fluid conduit and the wellbore wall,such as a geological formation, coupling the two. In addition, the solidcement provides a physical barrier that prohibits gases and liquids frommigrating from one side of the solid cement to the other via thewellbore annulus. This fluid isolation does not permit fluid migrationuphole of the solid cement through the wellbore annulus.

Displacing wellbore fluid for primary cementing operations is similar toestablishing circulation in the wellbore fluid with a drilling mud. Anamount of cement slurry introduced into the wellbore through a firstfluid conduit induces fluid flow in the wellbore and displaces anequivalent amount of wellbore fluid to the surface through a secondfluid conduit. In such an instance, the wellbore fluid includes aportion of the wellbore fluid previously contained in the wellborebefore cement introduction as well as the amount of the introducedcement slurry.

As previously stated in this description, high density cements, whichinclude cements with a density ranging from 140 pounds per cubic foot(pcf to 170 pcf are commonly used in wellbores because the high densitycements are less porous than low density cements, which include cementswith a density ranging from 65 pcf to 139 pcf, and, therefore, reducethe amount of undesirable components, such as undesirable hydrocarbons,that migrate from the geological formation into the tubular. However,internal pressure within the wellbore can cause tensile stress on thecement structure within the wellbore. Because the compressive strengthof the cement is around ten times greater than the tensile strength ofthe cement, lesser tensile stresses placed on the cement component maybe just as detrimental to the cement component as much greatercompressive stresses. These tensile stresses can cause damage, such ascracks or fractures, to form in the cement structure. High densitycements may be particularly prone to damage because the reduced porousstructure of the high density cements, when compared to low densitycements, may allow less flexibility in the cement structure. Once thecement structure is damaged, undesired component, such as undesiredhydrocarbons, may migrate from the geological formation into thetubular.

This migration of components into the tubular can cause contamination ofthe wellbore product when the wellbore is in use, which required costlyand time-consuming separations. Additionally, damage to the cementstructure may allow components, such as undesired hydrocarbons tomigrate into the tubular after the wellbore is abandoned. Thesecomponents can then move through the tubular and exit the wellbore,which may be detrimental to the environment.

In view of these previously discussed issues that can occur when thecement structure in the wellbore is damaged, the presently describedflexible additives may be added to the cement structure. Without beingbound to any particular theory, it is believed that the flexibleadditive forms a polymeric matrix within the cement structure thatprovides the cement structure with flexibility. This flexibility allowsthe cement structure to better withstand the tensile stresses caused byinternal pressure in the wellbore.

Without being bound by theory, it is believed that the flexibleadditives presently described may have a beneficial effect with respectto one or more of the problems with cement damage, as described. Aspreviously described in the present description, the flexible additivemay comprise a hydrophilic polyolefin fiber, such as, in embodiments,2,6-di-tert-butyl-p-cresol. It should be understood that whileembodiments of flexible additives presently described include thesecomponents, other components may be included in a flexible additive forvarious functional reasons, and it is contemplated that additionalcomponents may be included in the flexible additives presentlydescribed.

As presently described, flexible additives according to embodimentscomprise, consist, or consist essentially of one or more hydrophilicpolyolefin fibers. In one or more embodiments, the flexible additivecomprises, consists, or consists essentially of2,6-di-tert-butyl-p-cresol.

According to one or more embodiments, the flexible additive may have adensity from 1000 kilograms per cubic meter (kg/m³) to 1700 kg/m³, suchas from 1050 kg/m³ to 1700 kg/m³, from 1100 kg/m³ to 1700 kg/m³, from1150 kg/m³ to 1700 kg/m³, from 1200 kg/m³ to 1700 kg/m³, from 1250 kg/m³to 1700 kg/m³, from 1300 kg/m³ to 1700 kg/m³, from 1350 kg/m³ to 1700kg/m³, from 1400 kg/m³ to 1700 kg/m³, from 1450 kg/m³ to 1700 kg/m³,from 1500 kg/m³ to 1700 kg/m³, from 1550 kg/m³ to 1700 kg/m³, from 1600kg/m³ to 1700 kg/m³, or from 1650 kg/m³ to 1700 kg/m³. In otherembodiments, the density of the flexible additive is from 1000 kg/m³ to1650 kg/m³, such as from 1000 kg/m³ to 1600 kg/m³, from 1000 kg/m³ to1550 kg/m³, from 1000 kg/m³ to 1500 kg/m³, from 1000 kg/m³ to 1450kg/m³, from 1000 kg/m³ to 1400 kg/m³, from 1000 kg/m³ to 1350 kg/m³,from 1300 kg/m³ to 1700 kg/m³, from 1000 kg/m³ to 1250 kg/m³, from 1000kg/m³ to 1200 kg/m³, from 1000 kg/m³ to 1150 kg/m³, from 1000 kg/m³ to1100 kg/m³, or from 1050 kg/m³ to 1700 kg/m³.

The specific gravity of the flexible additive according to one or moreembodiments may be from 2.0 to 3.0, such as from 2.1 to 3.0, from 2.2 to3.0, from 2.3 to 3.0, from 2.4 to 3.0. from 2.5 to 3.0, from 2.6 to 3.0,from 2.7 to 3.0, from 2.8 to 3.0, or from 2.9 to 3.0. In otherembodiments, the specific gravity of the flexible additive is from 2.0to 2.9, such as from 2.0 to 2.8, from 2.0 to 2.7, from 2.0 to 2.6, from2.0 to 2.5, from 2.0 to 2.4, from 2.0 to 2.3, from 2.0 to 2.2, from 2.0to 2.1. In still other embodiments, the specific gravity of the flexibleadditive is about 2.4, about 2.5, about 2.6, about 2.7, or about 2.8.Although not being bound to any particular theory, the density andspecific gravity of the flexible additive presently described isbelieved to allow the flexible additive to move freely and bewell-dispersed in the cement slurry so that the flexible additive may beuniformly present in the cement slurry and the cement structure once itis cured. A well-dispersed flexible additive allows the flexibleadditive to be present at or near any position in the cement structure,which allows for well-distributed polymeric matrix within the curedcement structure. Accordingly, the well-dispersed flexible additive mayprovide flexibility and durability to a cement structure such that thecement structure can withstand tensile stresses caused by internalpressure of the wellbore.

It should be understood that the presently described properties of theflexible additive may, in embodiments, not be uniformly present in theflexible additive. For instance, in embodiments where multiplehydrophilic polyolefin fibers are added to the flexible additive, eachtype of hydrophilic polyolefin fiber may have its own density andspecific gravity. Accordingly, not every hydrophilic polyolefin fiber inthe flexible additive will have the same properties. Although, inembodiments, each component of the flexible additive may have propertieswithin the ranges presently described. In some embodiments, hydrophilicpolyolefin fibers can include rubbers, polyacrylonitrile fibers, andmixtures thereof.

The flexible additive may additionally include one or more viscosifiers.The viscosifier induces rheological properties (that is, thickening) inthe flexible additive composition that supports particle suspension andhelps to prevent losses into the other fluids or the geologicalformation. The viscosifier can include biological polymers, clays,ethoxylated alcohols and polyether glycols. Biological polymers andtheir derivatives include polysaccharides, including xanthan gums, welangums, guar gums, cellulose gums, corn, potato, wheat, maize, rice,cassava, and other food starches, succinoglycan, carrageenan, andscleroglucan and other intracellular, structural and extracellularpolysaccharides. Biological polymers also include chemically modifiedderivatives such as carboxymethyl cellulose, polyanionic cellulose andhydroxyethyl cellulose (HEC) and forms of the polymers suspended insolvents. Clays and their derivatives include bentonite, sepiolite,attapulgite, and montmorillionite. Polyalklyene glycols includepolyethylene glycols and polypropylene glycols, which are macromoleculeswith a series of internal ether linkages. Polyalklyene glycols arecapable of dissolving in water and have a greater impact on viscositywith greater molecular weight.

The viscosifier can also include a viscosity thinner. A viscositythinner reduces flow resistance and gel development by reducingviscosity of the flexible additive. Thinners comprising large molecularstructures can also act as fluid loss additives. The functional groupsof the viscosity thinners can act to emulsify oils and hydrocarbonspresent in the aqueous phase. Chemically modified viscosity thinners canattract solids and particles in the flexible additive and disperse suchparticles, the dispersion of particles preventing any increase inviscosity of the spacer fluid due to aggregation.

Polyphenolics, which include tannins, lignins, and humic acids, andchemically modified polyphenolics are useful viscosity thinners. Tanninsand their chemically modified derivatives can either originate fromplants or be synthetic. Examples of plant-originating tannins includetannins from pine, redwood, oak, and quebracho trees and bark; grapesand blueberries; and walnuts and chestnuts.

The flexible additive composition may also include one or more weightingagents. The weighting agent provides the flexible additive with theproper density profile. The proper weighing of the flexible additivecomposition relative to the cement slurry ensures that the flexibleadditive composition does not separate from the cement slurry. Weightingagents include sand, barite (barium sulfate), hematite, fly ash, silicasand, ilmenite, manganese oxide, manganese tetraoxide, zinc oxide,zirconium oxide, iron oxide and fly ash. According to one embodiment,the weighting agent for the flexible additive composition is barite.

A cement slurry may include water and a cement precursor, in addition toa presently described flexible additive. The cement slurry presentlydescribed may include silica sand with an average particle size from 80to 120 microns, such as from 90 to 110 microns, or about 100 microns.

The cement slurry of the present description may include water, a cementprecursor material, and the presently described flexible additive. Thecement precursor material may be any suitable material which, when mixedwith water, can be cured into a cement. The cement precursor materialmay be hydraulic or non-hydraulic. A hydraulic cement precursor materialrefers to a mixture of limestone, clay and gypsum burned together underextreme temperatures, such as temperatures up to 500° F. that may beginto harden instantly to within a few minutes while in contact with water.A non-hydraulic cement precursor material refers to a mixture of lime,gypsum, plasters and oxychloride. A non-hydraulic cement precursor maytake longer to harden or may require drying conditions for properstrengthening, but often is more economically feasible. A hydraulic ornon-hydraulic cement precursor material may be chosen based on thedesired application of the cement slurry of the present description.While hydraulic cement may be more commonly utilized in drillingapplications, it should be understood that other cements arecontemplated. In some embodiments, the cement precursor material may bePortland cement precursor. Portland cement precursor is a hydrauliccement precursor (cement precursor material that not only hardens byreacting with water but also forms a water-resistant product) producedby pulverizing clinkers, which contain hydraulic calcium silicates andone or more of the forms of calcium sulphate as an inter groundaddition. In embodiments, Portland cement was used, such as Portlandcement Class G or Portland cement Class H. The Setting or thickeningtime, according to embodiments, is in a range from 30 minutes to 15hours, such as from 5 hours to 10 hours. In embodiments, the curingtemperature range is from 70 degrees Fahrenheit (° F.) to 500° F., suchas from 200° F. to 300° F.

The cement precursor material may include one or more of calciumhydroxide, silicates, oxides, belite (Ca₂SiO₅), alite (Ca₃SiO₄),tricalcium aluminate (Ca₃Al₂O₆), tetracalcium aluminoferrite(Ca₄Al₂Fe₂O₁₀), brownmilleriate (4CaO.Al₂O₃.Fe₂O₃), gypsum (CaSO₄.2H₂O)sodium oxide, potassium oxide, limestone, lime (calcium oxide),hexavalent chromium, calcium alluminate, other similar compounds, andcombinations of these. The cement precursor material may includePortland cement, siliceous fly ash, calcareous fly ash, slag cement,silica fume, any known cement precursor material or combinations of anyof these. In one or more embodiments, the cement precursor comprisessilica sand.

In some embodiments, the cement slurry may contain from 50% by weight ofthe cement (BWOC) to 90% BWOC of the cement precursor material based onthe total weight of the cement slurry. For instance, the cement slurrymay contain from 50% BWOC to 80% BWOC, from 50% BWOC to 70% BWOC, orfrom 50% BWOC to 60% BWOC. The cement slurry may contain from 55% BWOCto 90% BWOC, from 60% BWOC to 90% BWOC, from 70% BWOC to 90% BWOC, orfrom 75% BWOC to 90% BWOC of the cement precursor material.

Accordingly, in embodiments, the cement slurry may contain from 10% BWOCto 50% BWOC of the flexible additive based on the total weight of thecement slurry. For instance, the cement slurry may contain from 10% BWOCto 40% BWOC, from 10% BWOC to 30% BWOC, or from 10% BWOC to 20% BWOC.The cement slurry may contain from 15% BWOC to 50% BWOC, from 20% BWOCto 50% BWOC, from 30% BWOC to 50% BWOC, or from 40% BWOC to 50% BWOC ofthe flexible additive.

Water may be added to the cement precursor material to produce theslurry. The water may be distilled water, deionized water, brackishwater, formation water, produced water, raw seawater, filtered seawater,or tap water. In some embodiments, the water may contain additives orcontaminants. For instance, the water may include freshwater orseawater, natural or synthetic brine, or salt water. In someembodiments, salt or other organic compounds may be incorporated intothe water to control certain properties of the water, and thus thecement slurry, such as density. Without being bound by any particulartheory, increasing the saturation of water by increasing the saltconcentration or the level of other organic compounds in the water mayincrease the density of the water, and thus, the cement slurry. Suitablesalts may include, but are not limited to, alkali metal chlorides,hydroxides, or carboxylates. In some embodiments, suitable salts mayinclude sodium, calcium, cesium, zinc, aluminum, magnesium, potassium,strontium, silicon, lithium, chlorides, bromides, carbonates, iodides,chlorates, bromates, formates, nitrates, sulfates, phosphates, oxides,fluorides, and combinations of these.

In some embodiments, the cement slurry may contain from 5% BWOC to 70%BWOC water based on the total weight of the cement slurry. In someembodiments, the cement slurry may contain from 5% BWOC to 50% BWOC,from 5% BWOC to 30% BWOC, 5% BWOC to 20% BWOC, from 5% BWOC to 10% BWOC,or from 10% BWOC to 70% BWOC, from 30% BWOC to 70% BWOC, or from 50%BWOC to 70% BWOC of water. The cement slurry may contain from 20% BWOCto 40% BWOC, or from 25% BWOC to 35% BWOC, such as 30% BWOC of waterbased on the total weight of the cement slurry.

The cement slurry presently described may also include an expansionadditive. The expansion additive is used to achieve good bonding withthe geological formation of the wellbore. As the cement dehydrates, itsvolume decreases, which causes a decreased bond between the cement and acasing or the cement and the geological formation. Expansion additivesincrease the volume of the cement and can improve the boding as thecement dehydrates. At wellbore temperatures of 140° F. or greater, atleast one of MgO, CaO, and mixtures thereof may be used as the expansionadditive in the cement slurry. However, at temperatures less than 140°F., MgO does not expand quickly enough to provide adequate binding tothe geological formation. Accordingly, at wellbore temperatures lessthan 140° F., crystalline SiO₂ may be used as the expansion additivebecause it expands more quickly than MgO. In one or more embodiments,D174 manufactured by Schlumberger may be used as a low-temperatureexpansion additive (such as, at temperatures less than 230° F.),Halliburton Micro bond L may be used as a low-temperature expansionadditive (such as, at temperatures less than 230° F.), Halliburton Microbond HT may be used as a high-temperature expansion additive (such as,at temperatures greater than 230° F.), and Schlumberger D 176 can beused as a high-temperature expansion additive (such as, at temperaturesgreater than 230° F.).

In some embodiments, the cement slurry may contain a weighting agent.Weighting agents may include, for example, magnesium tetraoxide (Mn₃O₄),hematite, calcium carbonate (CaCO₃), and barium sulfate (BaSO₄), andmixtures thereof. In one or more embodiments, the weighting agent isMn₃O₄ because it can have a small particle size, spherical shape, andhigh specific gravity, which allows Mn₃O₄ to reduce solids loading andsettling compared to other weighting agents.

In some embodiments, the cement slurry may contain from 0.1% BWOC to 50%BWOC of the one or more additional additives, as subsequently described,based on the total weight of the cement slurry. For instance, the cementslurry may contain from 0.1% BWOC to 8% BWOC of the one or moreadditional additives, from 0.1% BWOC to 5% BWOC of the one or moreadditives, or from 0.1% BWOC to 3% BWOC of the one or more additives.The cement slurry may contain from 1% BWOC to 10% BWOC of the one ormore additives, from 1% BWOC to 8% BWOC, from 1% BWOC to 5% BWOC, orfrom 1% BWOC to 3% BWOC of the one or more additives. In someembodiments, the cement slurry may contain from 3% BWOC to 5% BWOC, from3% BWOC to 8% BWOC, from 3% BWOC to 10% BWOC, or from 5% BWOC to 10%BWOC of the one or more additives.

In some embodiments, the one or more additional additives may include adispersant containing one or more anionic groups. For instance, thedispersant may include synthetic sulfonated polymers, lignosulfonateswith carboxylate groups, organic acids, hydroxylated sugars, otheranionic groups, or combinations of any of these. Without being bound byany particular theory, in some embodiments, the anionic groups on thedispersant may be adsorbed on the surface of the cement particles toimpart a negative charge to the cement slurry. The electrostaticrepulsion of the negatively charged cement particles may allow thecement slurry to be dispersed and more fluid-like, improvingflowability. This may allow for one or more of the following: turbulenceat lesser pump rates; reduction of friction pressure when pumping;reduction of water content; and improvement of the performance of fluidloss additives.

In some embodiments, the one or more additional additives mayalternatively or additionally include a fluid loss additive. In someembodiments, the cement fluid loss additive may include non-ioniccellulose derivatives. In some embodiments, the cement fluid lossadditive may be hydroxyethylcellulose (HEC). In other embodiments, thefluid loss additive may be a non-ionic synthetic polymer (for example,polyvinyl alcohol or polyethyleneimine). In some embodiments, the fluidloss additive may be an anionic synthetic polymer, such as2-acrylamido-2-methylpropane sulfonic acid (AMPS) or AMPS-copolymers,including lattices of AMPS-copolymers. In some embodiments, the fluidloss additive may include bentonite, which may additionally viscosifythe cement slurry and may, in some embodiments, cause retardationeffects. Without being bound by any particular theory, the surfactantmay reduce the surface tension of the aqueous phase of the cementslurry, thus reducing the fluid lost by the slurry. Additionally, thecarboxylic acid may further reduce the fluid loss of the cement slurryby plugging the pores of the cement filter cake, minimizing space forthe water or other fluids to escape from the cement.

In some embodiments, the fluid loss additive may contain a carboxylicfatty acid having from 16 to 18 carbon atoms, which may be used incombination with the surfactant to reduce fluid loss in the cementslurry. The carboxylic fatty acid includes any acids having formula ROOHin which R is a saturated or unsaturated, linear, or branchedhydrocarbyl group having from 16 to 18 carbons, such as a hydrocarbylgroup having 16 carbons, 17 carbons, or 18 carbons. Examples of suitablecarboxylic fatty acids include palmitic acid, palmitoleic acid, vaccenicacid, oleic acid, elaidic acid, linoleic acid, α-linolenic acid,α-linolenic acid, stearidonic acid, and combinations thereof. Thesurfactant may be in accordance with any of the embodiments previouslydescribed. In some specific embodiments, the fluid loss additive maycontain a combination of an ethylene oxide condensate of branchedisotridecyl alcohol with a fatty acid having from 16 to 18 carbon atomsin the hydrocarbyl group.

Following introduction of the cement slurry into the wellbore, thecement slurry may form cement through curing. As used throughout thedescription, “curing” refers to providing adequate moisture, temperatureand time to allow the concrete to achieve the desired properties (suchas hardness) for its intended use through one or more reactions betweenthe water and the cement precursor material. Curing may be a passivestep where no physical action is needed (such as cement that cures inambient conditions when untouched). In contrast, “drying” refers tomerely allowing the concrete to achieve a moisture condition appropriatefor its intended use, which may only involve physical state changes, asopposed to chemical reactions. In some embodiments, curing the cementslurry may refer to passively allowing time to pass under suitableconditions upon which the cement slurry may harden or cure throughallowing one or more reactions between the water and the cementprecursor material. Suitable conditions may be any time, temperature,pressure, humidity, and other appropriate conditions known in the cementindustry to cure a cement composition. In some embodiments, suitablecuring conditions may be ambient conditions. Curing may also involveactively hardening or curing the cement slurry by, for instance,introducing a curing agent to the cement slurry, providing heat or airto the cement slurry, manipulating the environmental conditions of thecement slurry to facilitate reactions between the water and the cementprecursor, a combination of these, or other such means.

In some embodiments, curing may occur at a relative humidity of greaterthan or equal to 80% in the cement slurry and a temperature of greaterthan or equal to 50° F. for a time period of from 1 to 14 days. Curingmay occur at a relative humidity of from 80% to 100%, such as from 85%to 100%, or 90% to 100%, or from 95% to 100% relative humidity in thecement slurry. The cement slurry may be cured at temperatures of greaterthan or equal to 50° F., such as greater than or equal to 75° F.,greater than or equal to 80° F., greater than or equal to 100° F., orgreater than or equal to 120° F. The cement slurry may be cured attemperatures of from 50° F. to 250° F., or from 50° F. to 200° F., orfrom 50° F. to 150° F., or from 50° F. to 120° F. The cement slurry maybe cured for from 1 day to 14 days, such as from 3 to 14 days, or from 5to 14 days, or from 7 to 14 days, or from 1 to 3 days, or from 3 to 7days.

Once the cement slurry is cured, the cured cement constitutes a cementstructure within the wellbore. The cement structure will have variousproperties that indicate the physical strength and flexibility of thecement structure. For instance, Young's modulus measures the ratio ofthe stress (force per unit area) along an axis to the strain (ratio ofdeformation over initial length) along that axis. Thus, Young's moduluscan be used to show the elasticity or stiffness of the cement structurewithin the wellbore and gives insight into the tensile strength of thecement structure. Poisson's ratio is a measure of transverse strain toaxial strain, and measures the deformation capacity of the cementstructure. The greater the deformation capacity (that is, the greaterPoisson's ratio) the less likely the cement structure will be damaged astemperature and pressure changes within the wellbore. The Young'smodulus and Poisson's ratio were measure 10 days after curing the cementstructure, 20 days after curing the cement structure, and 30 days aftercuring the cement structure.

In one or more embodiments, the static Young's modulus of the cementstructure 10 days after curing is from 0.90×10⁶ pounds per square inch(psi) to 1.20×10⁶ psi, such as from 0.95×10⁶ psi to 1.20×10⁶ psi, from1.00×10⁶ psi to 1.20×10⁶ psi, from 1.05×10⁶ psi to 1.20×10⁶ psi, from1.10×10⁶ psi to 1.20×10⁶ psi, or from 1.15×10⁶ psi to 1.20×10⁶ psi. Inother embodiments, the static Young's modulus of the cement structure 10days after curing is from 0.90×10⁶ psi to 1.15×10⁶ psi, such as from0.90×10⁶ psi to 1.10×10⁶ psi, from 0.90×10⁶ psi to 1.05×10⁶ psi, from0.90×10⁶ psi to 1.00×10⁶ psi, or from 0.90×10⁶ psi to 0.95×10⁶ psi. Inone or more embodiments, the static Young's modulus of the cementstructure 20 days after curing is from 0.90×10⁶ psi to 1.20×10⁶ psi,such as from 0.95×10⁶ psi to 1.20×10⁶ psi, from 1.00×10⁶ psi to 1.20×10⁶psi, from 1.05×10⁶ psi to 1.20×10⁶ psi, from 1.10×10⁶ psi to 1.20×10⁶psi, or from 1.15×10⁶ psi to 1.20×10⁶ psi. In other embodiments, thestatic Young's modulus of the cement structure 20 days after curing isfrom 0.90×10⁶ psi to 1.15×10⁶ psi, such as from 0.90×10⁶ psi to 1.10×10⁶psi, from 0.90×10⁶ psi to 1.05×10⁶ psi, from 0.90×10⁶ psi to 1.00×10⁶psi, or from 0.90×10⁶ psi to 0.95×10⁶ psi. In one or more embodiments,the static Young's modulus of the cement structure 30 days after curingis from 0.90×10⁶ psi to 1.20×10⁶ psi such as from 0.95×10⁶ psi to1.20×10⁶ psi, from 1.00×10⁶ psi to 1.20×10⁶ psi, from 1.05×10⁶ psi to1.20×10⁶ psi, from 1.10×10⁶ psi to 1.20×10⁶ psi, or from 1.15×10⁶ psi to1.20×10⁶ psi. In other embodiments, the static Young's modulus of thecement structure 30 days after curing is from 0.90×10⁶ psi to 1.15×10⁶psi, such as from 0.90×10⁶ psi to 1.10×10⁶ psi, from 0.90×10⁶ psi to1.05×10⁶ psi, from 0.90×10⁶ psi to 1.00×10⁶ psi, or from 0.90×10⁶ psi to0.95×10⁶ psi.

In one or more embodiments, the static Poisson's ratio of the cementstructure 10 days after curing is from 0.120 psi to 0.140 psi, such asfrom 0.125 psi to 0.140 psi, from 0.130 psi to 0.140 psi, or from 0.135psi to 0.140 psi. In other embodiments, the static Poisson's ratio ofthe cement structure 10 days after curing is from 0.120 psi to 0.135psi, from 0.120 psi to 0.130 psi, or from 0.120 psi to 0.125 psi. In oneor more embodiments, the static Poisson's ratio of the cement structure20 days after curing is from 0.110 psi to 0.130 psi, such as from 0.115psi to 0.130 psi, from 0.120 psi to 0.130 psi, or from 0.125 psi to0.130 psi. In other embodiments, the static Poisson's ratio of thecement structure 20 days after curing is from 0.110 psi to 0.125 psi,from 0.110 psi to 0.120 psi, or from 0.110 psi to 0.115 psi. In one ormore embodiments, the static Poisson's ratio of the cement structure 30days after curing is from 0.190 psi to 0.210 psi, from 0.195 psi to0.210 psi, from 0.200 psi to 0.210 psi, or from 0.205 psi to 0.210 psi.In other embodiments, the static Poisson's ratio of the cement structure30 days after curing is from 0.190 psi to 0.205 psi, from 0.190 psi to0.200 psi, or from 0.190 psi to 0.195 psi.

The cement structure may, in embodiments, have a density from 1.80 gramsper cubic centimeter (g/cc) to 2.20 g/cc, such as from 1.85 g/cc to 2.15g/cc from 1.90 g/cc to 2.10 g/cc, from 1.95 g/cc to 2.05 g/cc, or about2.00 g/cc. the cement structure may include pores that allow undesirablecomponents, such as undesirable hydrocarbons, to migrate from thegeological formation into the tubular through the cement structure.However, if the density of the cement structure exceeds 170 pounds percubic foot (pcf), the cement structure may not have enough elasticity tosurvive exposure to tensile stresses caused by internal pressures in thewellbore. As presently described, flexible additives according toembodiments may be used in a wide array of cements have many densities.

A first aspect includes, a cement slurry comprising: 50% to 90% BWOC ofa cement precursor material based on a total weight of the cementslurry; and from 10% to 50% BWOC of a flexible additive based on thetotal weight of the cement slurry, where the flexible additive compriseshydrophilic polyolefin fibers or 2,6-di-tert-butyl-p-cresol.

A second aspect includes the cement slurry of the first aspect, wherethe flexible additive comprises 2,6-di-tert-butyl-p-cresol.

A third aspect includes the cement slurry of any one of the first andsecond aspects, where the flexible additive consists essentially of2,6-di-tert-butyl-p-cresol.

A fourth aspects includes the cement slurry of the first aspect, wherethe flexible additive comprises hydrophilic polyolefin fibers selectedfrom the group consisting of rubbers, polyacrylonitrile fibers, andmixtures thereof.

A fifth aspect includes the cement slurry of any one of the first tofourth aspects, where the flexible additive has a density from 1000kg/m³ to 1700 kg/m³.

A sixth aspect includes the cement slurry of any one of the first tofifth aspects, where the flexible additive has a specific gravity from2.0 to 3.0.

A seventh aspect includes the cement slurry of any one of the first tosixth aspects, where the cement slurry comprises from 10% to 30% BWOC ofthe flexible additive based on the total weight of the cement slurry.

An eighth aspect includes a wellbore cementing system comprising: atubular positioned in a wellbore such that an annulus is formed betweena geological formation and the tubular; and a cement structurepositioned in at least a portion of the annulus, where the cementstructure comprises from 10% to 50% BWOC of a flexible additive, andwhere the flexible additive comprises hydrophilic polyolefin fibers or2,6-di-tert-butyl-p-cresol.

A ninth aspect includes the wellbore cementing system of the eighthaspect, where the flexible additive comprises2,6-di-tert-butyl-p-cresol.

A tenth aspect includes the wellbore cementing system of any one of theeighth and ninth aspects, where the flexible additive consistsessentially of 2,6-di-tert-butyl-p-cresol.

An eleventh aspect includes the wellbore cementing system of the eighthaspect, where the flexible additive comprises hydrophilic polyolefinfibers selected from the group consisting of rubbers, polyacrylonitrilefibers, and mixtures thereof.

A twelfth aspect includes the wellbore cementing system of any one ofthe eighth to eleventh aspects, where the flexible additive has adensity from 1000 kg/m³ to 1700 kg/m³.

A thirteenth aspect includes the wellbore cementing system of any one ofthe eighth to twelfth aspects, where the flexible additive has aspecific gravity from 2.0 to 3.0.

A fourteenth aspect includes the wellbore cementing system of any one ofthe eighth to thirteenth aspects, where the cement structure comprisesfrom 10% to 30% BWOC of the flexible additive based on a total weight ofthe cement structure.

A fifteenth aspect includes the wellbore cementing system of any one ofthe eighth to fourteenth aspects, where the cement structure has aYoung's modulus 10 days after curing from 0.90×10⁶ psi to 1.20×10⁶ psi.

A sixteenth aspect includes the wellbore cementing system of any one ofthe eighth to fifteenth aspects, where the cement structure has aYoung's modulus 20 days after curing from 0.90×10⁶ psi to 1.20×10⁶ psi.

A seventeenth aspect includes the wellbore cementing system of any oneof the eighth to sixteenth aspects, where the cement structure has aYoung's modulus 30 days after curing from 0.90×10⁶ psi to 1.20×10⁶ psi.

An eighteenth aspect includes the wellbore cementing system of any oneof the eighth to seventeenth aspects, where the cement structure has aPoisson's ratio 10 days after curing from 0.120 psi to 0.140 psi.

A nineteenth aspect includes the wellbore cementing system of any one ofthe eighth to eighteenth aspects, where the cement structure has aPoisson's ratio 20 days after curing from 0.110 psi to 0.130 psi.

A twentieth aspect includes the wellbore cementing system of any one ofthe eighth to nineteenth aspects, where the cement structure has aPoisson's ratio 30 days after curing from 0.190 psi to 0.210 psi.

Examples

The following example illustrates one or more features of the presentdescription. It should be understood that these examples are notintended to limit the scope of the description or the appended claims inany manner.

A cement slurry was tested for rheology, thickening time, fluid loss,free water, sedimentation, expansion performance, and mechanicalproperties in order to evaluate the performance of cement slurry. Thecement slurry included silica sand with an average particle size of 100microns, crystalline silica expansion additives, and a2,6-di-tert-butyl-p-cresol flexible additive. Two sizes of crystallinesilica were used; Schlumberger micro fine silica (D178) and Schlumbergercoarse silica (D030).

The cement slurry formulation was prepared in the lab using the standardAmerican Petroleum Institute (API) blender. The maximum speed usedduring slurry preparation was 12,000 rotations per minute (rpm). Theslurry was mixed in the blender for 15 seconds at 4,000 rpm and 35seconds at 12,000 rpm. The slurry was then conditioned in theatmospheric consistometer before obtaining the rheological measurements.A Fann viscometer (Model-35) was utilized to measure the slurry apparentviscosity.

The prepared slurry was then poured into API standard High Pressure/HighTemperature (HP/HT) consistometer slurry cup for a thickening time test,which is important to evaluate the pumpability of the cement slurry.

As in API Recommended testing 10-B2, a free water test was used tomeasure water separation by using 250 milliliter (ml) graduated cylinderin the cement slurry for 2 hours. Settling was measured by comparingdensities of different sections of the cement column cured. Thecylindrical shaped cell, used to cure the cement formula for settlingtest, had a diameter of 1.4″ and length of 12″. Sections of 2″ long weretaken from different parts of the cement column sample. The cementslurry was cured at 8000 psi and 300° F. for at least 3 days.

To measure expansion, an annular expansion ring test was used to measurelinear expansion under condition of free access to water. Free access towater means an open system. An annular expansion mold was used tosimulate the annulus of the well. The cement slurry was poured into theannular space in the mold and then the mold was placed into water bathor a pressurized curing chamber. Water was in contact with the slurryduring the entire curing process as in API Recommended testing 10-B2.The diameter increased if the cement expanded.

A composition and properties of the cement slurry is provided in Table 1and properties of the cement slurry are provided in Tables 2 and 3. InTable 2, the rheology of the cement slurry was measured using a standardviscometer. Ramp up in Table 2 indicates increasing rpm to 3, 6, 100,200, and 300. Ramp down in Table 2 indicates decreasing rpm from 300 to200, 100, 6, and 3. In Table 3 the thickening time of the slurry ismeasured by pouring the slurry into a cylinder with 0 degree inclination(a vertical cylinder) and heating to 80° F. for several hours. The solidsedimentation at the bottom section of the cylinder is observed. Nosedimentation means that cement will have good quality at both the topand the bottom of the cement structure. The API fluid loss is a testthat measures the volume of filtrate of the cement slurry at hightemperature and pressure. As used in this example, high temperature isin a range from 180° F. to 500° F., and high pressure is in a range from5,000 psi to 20,000 psi. In Table 3, the “BC” is the Bearden unit ofconsistency, and an acceptable fluid level is 0 ml/250 ml at atmosphericconditions. In this example, the components listed in Table 1 were allmanufactured by Schlumberger, and the Schlumberger material number islisted in Table 1.

TABLE 1 Cement Slurry: Component Concentration Unit of Measure Freshwater 5.267 gps Flexible Additive (D196) 20.300 % BWOC Silica (D030)10.200 % BWOC Silica (D178) 30.900 % BWOC Weighting Agent (D157) 56.200% BWOC High Temp. Expansive Agent (D176) 3.000 % BWOC Antifoam (D175)0.062 gps Dispersant (D065) 0.660 % BWOC Fluid Loss (D167) 0.330 % BWOCRetarder (D800) 0.700 % BWOC GASBLOK (D600G) 1.452 gps GASBLOKstabilizer (D135) 0.207 gps

TABLE 2 Rheology of the Cement Slurry: Rheology at 81 F. Ramp up Rampdown RPM (measurement) (measurement) Average 300 128 128 128 200 89 9592 100 53 57 55 60 37 40 39 30 25 28 26 6 15 14 15 3 13 13 13

TABLE 3 Properties of Cement Slurry: Thickening time Consistency Time 67Bc 7:35 hrs Free Fluid 0 ml/250 ml in 2 hrs 80 F., 0 deg inclination Nosedimentation Fluid loss API fluid loss 76 ml 14 min, 186 F., and 1000psi

Single stage triaxial tests were performed on 13 dry cement core plugswith lengths ranging between 2.997 and 3.020 inches and having adiameter between 1.490 and 1.510 inches to measure static and dynamicproperties through ultrasonic and shear velocities. These propertieswere determined at a confining pressures of 1 megapascals (MPa) (1MPa=145.038 psi) and included the Young's modulus, the Poisson's Ratio,the Peak Strength.

During each test performed, a series of ultrasonic measurements anddynamic moduli were computed. The final dynamic moduli of a plug weretaken as the average of the moduli computed at each ultrasonic velocitymeasurement.

Sample Preparation included the following steps: (1) cement core plugformulation was selected and drilled; (2) surfaces of the parallel endfaces were grinded until they became flat to within 0.001 inches; and(3) the plug was jacketed and positioned so that two end caps equippedwith velocity transducers could be placed on the ends of the samplewhile a coupling medium was set between the plug flat surfaces and thetransducer.

After completing the sample preparation as per the procedure in thepreceding paragraph, the plug was equipped and loaded onto the testingframe as follows: (a) the jacket was clamped to the transducers fromboth ends to allow for the hydrostatic confining pressure around thesample to be applied; (b) radial and axial limited variable differentialtransformers (LVTD) were positioned around and along the sample tomeasure radial and axial displacements respectively; and (c) confiningpressure was applied hydrostatically around the sample. The confiningpressures were selected to simulate the stress condition in the vicinityof the wellbore.

For this example single stage triaxial tests at low confining pressureswere conducted. The dynamic elastic properties were determinedsimultaneously with the static properties using ultrasonic measurements.The static properties are required for many petroleum engineeringapplications; however, dynamic data are often collected in the field andtherefore the necessary calibration must be obtained to design specifictreatments related to wellbore stability, hydraulic fracturing, and sandcontrol.

To perform dynamic measurements (ultrasonic velocity measurements) theend caps of the core sample were equipped with ultrasonic transducersand receivers which can generate and detect, respectively, bothcompressional and shear waves. One transducer was a transmitter whichwas excited to induce an ultrasonic wave at a frequency of 700 kilohertz(kHz) and the other one was a receiver. In this example the velocitiesof these waves were used to compute the dynamic Young's modulus andPoisson's ratio.

Mechanical Properties Simulation

Young's modulus E characterizes the material's longitudinal deformationunder uniaxial loading, such as along an axis when opposing forces areapplied along that axis. Transverse deformation is quantified with thePoisson's ratio υ, which is the ratio between transverse and axialdeformation. A Poisson's ratio equal to 0.5 means the material isincompressible. Conventional cements have a Poisson's ratio ofapproximately 0.15.

Results of the mechanical properties are shown in Table 4.

TABLE 4 Cement slurry (105 pcf) mechanical test results measuredaccording to ASTM D2850 and D4767 Standard Test Methods. Static DynamicSat. bulk Confining Young's Young's Static Dynamic density pressuremodulus modulus Poisson's Poisson's Peak Sample (g/cc) (psi) (psi) (psi)ration ration strength Remarks Comp. Ex. 1.99 447.5 2.52 × 10⁶ 2.64 ×10⁶ 0.168 0.308 12312 10 days 1 conventional cement 1 1.99 168   1.09 ×10⁶ 3.10 × 10⁶ 0.129 0.305  5203 10 days durable cement 2 1.99 472  1.10 × 10⁶ 1.65 × 10⁶ 0.118 0.32   4415 20 days durable cement 3 1.99477   1.08 × 10⁶ 1.89 × 10⁶ 0.205 0.298  4924 30 days durable cement

Having described the subject matter of the present description in detailand by reference to specific embodiments, it is noted that the variousdetails described in this description should not be taken to imply thatthese details relate to elements that are essential components of thevarious embodiments described in this description, even in cases where aparticular element is illustrated in each of the drawings that accompanythe present description. Rather, the claims infra should be taken as thesole representation of the breadth of the present description and thecorresponding scope of the various embodiments described in thisdescription. Further, it should be apparent to those skilled in the artthat various modifications and variations can be made to the describedembodiments without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various described embodimentsprovided such modification and variations come within the scope of theclaims recited infra and their equivalents.

It should be understood that any two quantitative values assigned to aproperty may constitute a range of that property, and all combinationsof ranges formed from all stated quantitative values of a given propertyare contemplated in this description. It should be appreciated thatcompositional ranges of a chemical constituent in a composition orformulation should be appreciated as containing, in some embodiments, amixture of isomers of that constituent. It should be appreciated thatthe examples supply compositional ranges for various compositions, andthat the total amount of isomers of a particular chemical compositioncan constitute a range.

As used in the Specification and appended Claims, the singular forms“a”, “an”, and “the” include plural references unless the contextclearly indicates otherwise. The verb “comprises” and its conjugatedforms should be interpreted as referring to elements, components orsteps in a non-exclusive manner. The referenced elements, components orsteps may be present, utilized or combined with other elements,components or steps not expressly referenced.

Where a range of values is provided in the Specification or in theappended Claims, it is understood that the interval encompasses eachintervening value between the upper limit and the lower limit as well asthe upper limit and the lower limit. The invention encompasses andbounds smaller ranges of the interval subject to any specific exclusionprovided. As used herein, the word “about” followed by a number includesthe stated number plus or minus two significant digits.

What is claimed is:
 1. A cement slurry comprising: 50% to 90% BWOC of acement precursor material based on a total weight of the cement slurry;from 10% to 50% BWOC of a flexible additive based on the total weight ofthe cement slurry; and water, where the flexible additive comprises2,6-di-tert-butyl-p-cresol.
 2. The cement slurry of claim 1, where theflexible additive consists essentially of 2,6-di-tert-butyl-p-cresol. 3.The cement slurry of claim 1, where the flexible additive has a densityfrom 1000 kg/m³ to 1700 kg/m³.
 4. The cement slurry of claim 1, wherethe flexible additive has a specific gravity from 2.0 to 3.0.
 5. Thecement slurry of claim 1, where the cement slurry comprises from 10% to30% BWOC of the flexible additive based on the total weight of thecement slurry.